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J. Am. Chem. SOC. 1995, 117, 7816-7817
Photoreactive Supramolecular Assemblies: Aggregation and Photoisomerization of Azohenzene Phospholipids in Aqueous Bilayers Xuedong Song, Jerry Perlstein, and David G. Whitten* Department of Chemisrp. Universiw of Rochester Rochester. New York 14627 Received April 3. 1995 The formation of aggregates is frequently encountered when amphiphiles incorporating chromophores such as dyes or aromatics are organized in microheterogeneous media such as thin films, bilayers, or microemulsions.” While in some cases aggregate formation may be driven by the self-organizing properties of the amphiphile. there exists the possibility that the presence of aggregates as either stable or metastable entities can have special consequences for the microscopic as well as macroscopic properties of the medium in which they are generated. This paper focuses on a demonstration of some of these effects when a fairly simple photoreactive aromatic chromophore. trans-azobenzene. is incorporated into a phospholipid structure which in turn can be used, either pure or in mixtures with saturated phospholipids, to form bilayer structures in aqueous media. We have shown that phospholipids containing a trans-stilbene chromophore in the fatty acid portion of a phosphatidylcholine form bilayer assemblies on dispersion in water in which the trans-stilbene chromophore is strongly agg~egated.~.~ The aggregate is characterized by a blue shift in the absorption spectrum, a red shift in fluorescence, and a strong induced circular dichroism (ICD) spectrum; studies of the aggregatemonomer or aggregate-dimer interconversion have established relatively small. integral values for the aggregation number. From these results and molecular simulations we have proposed a chiral cyclic “unit structure’’ for the stilbene aggregates.5 In other investigations a picture of a very similar structure for aggregates of squaraine dyes is emerging.“ The overall picture is one of fairly high stability and order in a relatively small aggregate unit such that even very large arrays of aggregate may be composed of a “mosaic” of small aggregates.’-’ We have synthesized a series of azobenzene phospholipids (APL) (1-3) shown in Scheme 1 which form aggregates similar to those of the stilbene but are somewhat more soluble in water and photochemically active. Absorption spectra of 1 in chloroform, water, and water with excess saturated phospholipid. dipalmitoyl phosphatidylcholine (DPPC). are similar to those obtained for related stilbeness and are assigned to the transazobenzene monomer, aggregate, and dimer, respectively. The isosbestic points observed upon mixing aqueous solutions of 1-3 with saturated phospholipids (DPPC or dimyristoyl phosphatidylcholine (DMPC)) suggest that the aggregates decompose directly into dimers; a Benesi-Hildebrand a n a l y s i ~indicates ~.~ that the aggregation numbers for 1-3 are 42, 3, and 3 azobenzene units, respectively. As was observed for the stilbene aggregates previously described,’ the azobenzene aggregates, (IINekahisa. H.: Fukadn. K.; Mobius. D.:Kuhn, H. 1. Pbw. Chem. 1986. YO. 6144-6148. (21 Evans. C. E.: Bohn. P. W. J. Am. Clwn. Sor. 1993. 115. 3306771 ......I
( 3 ) Whitten. D. G. Arc. Chcm. R r i . 1993. 2h. 502-509. (41 Song. X.: Geiger. C.: Furman. I.: Whirten. D. G. J. Am. Chtm. Sot, 1994. 116. 4103-4104 ( S I Song. X.: Geiger. C.: Leinhor. U.: Perlslein. 1.: Whitren, D. G. J. A m Chott. .Sw. 1994. 116. 10340-10341. (h1,Chm. H.: Law. K. Y.: Perlsrein. I.: Whitren. D. G.J. Am. Chem. .%,
